Vehicular trailer angle detection system

ABSTRACT

A vehicular trailer angle detection system includes a camera disposed at a rear portion of a vehicle. The system determines, via processing of frames of image data captured by the camera, features of a trailer present rearward of the vehicle and hitched to the vehicle by determining features that have similar position changes between a current frame of image data captured by the camera and a previous frame of image data captured by the camera. Responsive to movement of the trailer relative to the vehicle, and via processing of captured frames of image data, the system tracks determined features over multiple captured frames of image data for different positions of the trailer relative to the vehicle. The system determines angle of the trailer relative to the vehicle based at least in part on tracking of determined features of the trailer over multiple captured frames of image data.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent application Ser. No. 17/249,120, filed Feb. 22, 2021, now U.S. Pat. No. 11,651,510, which is a continuation of U.S. patent application Ser. No. 16/779,819, filed Feb. 3, 2020, now U.S. Pat. No. 10,930,004, which is a continuation of U.S. patent application Ser. No. 15/928,128, filed Mar. 22, 2018, now U.S. Pat. No. 10,552,976, which claims the filing benefits of U.S. provisional application Ser. No. 62/518,765, filed Jun. 13, 2017, and U.S. provisional application Ser. No. 62/474,646, filed Mar. 22, 2017, which are hereby incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates generally to a vehicle vision system for a vehicle and, more particularly, to a vehicle vision system that utilizes one or more cameras at a vehicle to assist a driver of the vehicle in maneuvering the vehicle with a trailer.

BACKGROUND OF THE INVENTION

Use of imaging sensors in vehicle imaging systems is common and known. Examples of such known systems are described in U.S. Pat. Nos. 5,949,331; 5,670,935 and/or 5,550,677, which are hereby incorporated herein by reference in their entireties.

In some systems, when attaching a trailer to the vehicle, the driver has to enter its properties to put the trailer driving aid system into a position to properly calculate the driving aids overlays, when backing up with a trailer attached. Examples of trailer detection systems are described in U.S. Pat. Nos. 9,296,422 and 8,930,140, and U.S. Publication No. US-2014-0160276, which are hereby incorporated herein by reference in their entireties.

SUMMARY OF THE INVENTION

The present invention provides a driver assistance system or vision system or imaging system or trailer angle detection system for a vehicle that utilizes one or more cameras (preferably one or more CMOS cameras) to capture image data representative of images exterior of the vehicle, with the vehicle towing a trailer, and with the system determining the angle of the trailer relative to the vehicle. The trailer angle detection system determines the presence of a trailer at the rear of the vehicle and attached at the vehicle and extracts feature vectors of the trailer. Responsive to movement of the vehicle and trailer, the trailer angle detection system tracks feature vectors for different positions of the trailer and calculates an angle of the trailer responsive to detection of tracking of feature vectors the trailer in the field of view of the camera.

These and other objects, advantages, purposes and features of the present invention will become apparent upon review of the following specification in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a vehicle with a vision system that incorporates a camera for capturing image data representative of a trailer being towed by the vehicle;

FIG. 2 is a plan view of the vehicle and trailer, showing the trailer angle of the trailer relative to the vehicle;

FIG. 3 is a block diagram of the trailer angle detection system of the present invention;

FIG. 4 is a flow chart of the image data processing in accordance with the trailer angle detection system of the present invention;

FIG. 5 is a context diagram of a trailer angle detection system of the present invention;

FIG. 6 is a flow chart showing operation of the trailer angle detection system of the present invention;

FIG. 7 is a diagram of the field of view that is processed for detecting the trailer, where features are detected only in the white regions, and not the black regions of the mask; and

FIG. 8 is a schematic showing a set of seven angular segments, each serving a range of 30 degrees.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings and the illustrative embodiments depicted therein, a rear vision system 10 for a vehicle 12 is operable to detect the angle of a trailer 14 that is pulled behind the vehicle 12 by using a rear view camera or multi-camera surround view system (FIG. 1 ). A camera 16 is mounted at the rear end portion of the pulling vehicle 12. An image processor (such as a digital processor or FPGA or DSP or ASIC or camera imager SOC or other suitable processor or processing means) is operable to process the image data captured by the camera and, responsive to such processing of captured image data, determines the angle of the trailer in relation to the pulling vehicle in real time. The system of the present invention is operable to determine the trailer angle as the vehicle is driven and turned, such as at corners or the like, along a road, as discussed below. The system is operable to measure the angle (see FIG. 2 ) between a tow vehicle and trailer using the rear view camera.

While reversing a vehicle that has trailer attached, it is difficult to control the trailer path especially for new or inexperienced drivers. As a result, the trailer may move toward an unintended direction which may lead to jackknifing the trailer. For controlled backing up of the vehicle with a trailer attached to it, the backup assist system should calculate the angle of trailer with respect to vehicle center axis. Some known systems use a target (such as a checker board or known pattern) that is placed on a horizontal surface of the trailer whereby the system calculates trailer position and angle by detecting and tracking the target. For the systems with a target, the driver/end user has to place the target on the trailer body and the system has limitations with visibility of the target. Any partial blockage of the target, such as because of weather (including snow), strong shadows, glare from light sources or the like, can introduce inaccuracies in the system.

The system of the present invention is operable to detect the region with radial motion (trailer body/tongue), without any target attached to the trailer body. The system extracts a feature vector from the radial motion region, which will be used over frames of captured image data to detect the trailer region, without using vehicle motion. The system detects the tip of the stationary region (hitch ball), which moves along with the vehicle, and calculates a single reference point or multiple reference points with which the trailer angle will be calculated. As shown in FIG. 3 , the rear camera captures image data representative of the scene with the target at the trailer, and the system processes the captured image data in accordance with the known mechanical parameters to determine the trailer angle and change rate.

The system thus performs trailer detection, such as via the process shown in FIG. 4 , to determine presence of a trailer at the rear of the vehicle. Using a motion pattern (horizontal and vertical) of pixels, the system detects the trailer region including trailer body and tongue that moves radially (relative to the hitch ball of the vehicle) in physical space, and the hitch ball region that moves along with the vehicle. The system extracts unique features from the trailer region that will be used over multiple frames of captured image data to match the trailer position within the frames of captured image data. The system confines the processing to a region of interest (ROI) created in the input image around the trailer in which the size and shape of the trailer features remains unchanged. This reduces processing time to acquire the trailer region.

The system acquires the trailer region within the ROI using features identified during the feature extraction phase. The system constructs/updates feature vectors for different positions of the trailer to limit the number of features. The system calculates centroid(s) for feature vectors with which the angle of the trailer will be measured. The system tracks the centroids with inputs of vehicle parameters including vehicle speed, acceleration and/or steering wheel angle. The system builds confidence to the tracker to avoid sudden drops in tracking and for a smaller ROI. The system is operable to detect the trailer angle with respect to the center line of the vehicle using the calculated centroid(s).

Therefore, the system may distinguish between the trailer region (including the trailer body and tongue) and the hitch ball positions using angular motion of the vehicle. The system measures the various distances including vehicle-to-hitch ball, hitch ball-to-centroid(s) of the trailer body and the camera-to-hitch ball. The system provides for detection of the presence of the trailer using motion patterns on different road surfaces including asphalt, concrete, dirt, gravel, grass, water, sand, snow and/or the like, and in different ambient conditions including daytime lighting conditions, dawn/dusk lighting conditions, nighttime lighting conditions with minimum light of 6 lux. The system determines/predicts the position of the trailer over time with the help of vehicle parameters and corresponding trailer location. The system thus avoids jackknifing the trailer by continuous detection and tracking of the trailer angle with respect to the vehicle center axis.

Therefore, the system of the present invention automatically identifies the presence of a trailer attached at the vehicle using angular motion of the trailer, and calculates the trailer position in physical space with respect to the hitch ball, so the user does not need to keep a target or known pattern on the trailer body. The trailer detection is independent of various climatic and ambient lighting conditions. The system extracts and constructs feature vectors for the trailer region at different positions, which reduces the processing time to detect trailer region. The system calculates multi point reference points (centroids) on the trailer region to reduce false positives and true negatives and to improve accuracy of the trailer angle calculation.

Optionally, and with reference to FIG. 5 , the trailer angle detection system receives as input the video stream from the towing vehicle's backup camera, and the vehicle CAN data, including current vehicle velocity and steering wheel angle, and whether the trailer brake lights are currently connected. The system is responsible for delivering the current angle of the trailer with respect to the towing vehicle, the rate of change of that angle, and the tracking or tracking status or current status of the trailer angle detection system. It also delivers the estimated length of the trailer, from hitch to trailer axle, and measures hitch length, at the time the trailer is first connected. As shown in FIG. 6 , the operation of trailer angle detection system is divided into four phases or states: an initialization state, a recognition of a newly attached trailer state, a new trailer calibration state (if needed), and a steady-state running state.

State 1: Initialization State:

When the trailer angle detect system starts, it first enters a phase where it needs to detect whether or not a trailer is attached at the rear of the vehicle. The trailer angle detect system identifies the presence of a trailer in the dynamic field of view (FOV) of the rear camera. The system measures the length of the hitch by locating the tip of the hitch. Pattern matching may be used to find the tip of the hitch, where recognizable patterns may include a circle, ellipse, square, rectangle, parabola and other polygonal shapes. This is the first step of locating the tip of the hitch using visual features.

The field of view of the rear camera is centered around the tip of the hitch. This allows the system to measure trailer angle by analyzing the movement of visual features from the video frames of captured image data. The field of view is shown in FIG. 7 , where features are detected only in the white regions, and not the black regions of the mask. The trailer angle detection system rotates the field of view around the tip of hitch based on an output of the tracker (the expected location of the trailer). The system also identifies if the trailer is connected to the towing vehicle by processing image data captured by the rear camera and by vehicle CAN signals. When a trailer is attached/connected, the trailer angle detection system proceeds to the Trailer Recognition phase.

State 2: Trailer Recognition:

The system may initially identify or recognize the trailer or determine whether or not the particular trailer at the rear of the vehicle is a trailer for which the trailer angle detection system is calibrated. If the system has been calibrated for that trailer, the system proceeds with the angle determination and tracking processes. If the system has not been calibrated for that trailer, the system proceeds first with a calibration stage, discussed below.

During the trailer recognition state, the system extracts the features that are a combination of patterns and other key feature points. These features are invariant to ambient light changes (for example, during nighttime lighting conditions, the system still recognizes a trailer that was calibrated during daytime lighting conditions), ground surface variations, and load changes on the trailer (for example, with/without a boat on a boat trailer, or with/without goods on a utility trailer).

These features are matched against stored features of each of the previously calibrated trailers and filtered, using the method of feature matching. For a particular calibrated trailer, the features are consolidated over the time, so that the system may learn in the background. This makes the trailer angle detection system intelligent.

The feature matching and filtering is done for all previously calibrated trailers, such that a previously calibrated trailer that best matches the current video frame is selected as the candidate “recognized” trailer. If the percentage of trackable features remaining for the candidate is less than a preset threshold, the trailer viewed in the current video image frame is deemed to not match any of the previously calibrated trailers, and the trailer angle detection system enters the new trailer calibration phase. If the candidate trailer matches at least as well as the threshold, the stored settings for that trailer, including the features to be tracked and the offset angle, are set as the reference image data, and the trailer angle detection system enters the steady state running phase.

State 3: New Trailer Calibration:

If the system passes through the trailer recognition phase without any previously calibrated trailer being recognized as matching the current trailer image, the system starts a sequence of steps to locate the hitch ball and capture a set of feature descriptors while the trailer is at an angle of 0 degrees for use as a reference during steady state running. The system may prune those features to eliminate ones that are unreliable (likely not actually on the trailer), and estimates the length of the trailer from hitch ball to axle.

To calibrate a new (not recognized) trailer, the system, after collecting data on the trailer, collects data during a calibration drive, while the vehicle is driven straight ahead for several seconds (such as greater than three seconds or greater than five seconds), with the steering angle close to zero degrees. After driving straight ahead, the driver executes a turn, traveling either forwards or backwards, such that the trailer angle exceeds ±5 degrees.

The system measures the length of the hitch by locating the tip of the hitch. Based on the motion of the trailer while the vehicle is driven straight ahead and especially when turned, the hitch ball location can be refined based on the motion of the trailer. This is the second step of locating the tip of the hitch.

The system may then estimate the trailer beam length, which is the distance between the tow ball and the center of the trailer axle. If a trailer has multiple axles then it is the distance between the tow ball and the center of multiple axles of trailer. The system estimates the length of the trailer beam using the trailer angle, vehicle dynamics, and camera and system parameters.

The trailer angle detection system finds or determines one or more zero degree reference frames of captured image data (and corresponding features) during a straight calibration drive.

All the collected data are then entered into the set of previously calibrated trailers, ready for use in future trailer recognitions. After the new trailer calibration phase is completed, the system enters the steady state running phase.

State 4: Steady State Running:

Once a trailer has been recognized or newly calibrated, the trailer angle detection system is ready to begin normal operation, referred to herein as steady state running. In steady state running, the trailer angle detection system starts producing an estimated trailer angle and angular rate of change, along with the system status, for each input video frame of captured image data.

Two different approaches are used in concert to calculate the trailer angle. A kinematic model of the movement of a car and trailer is used, and an analysis of the movement of visual features from the video frame, relative to a reference frame, is used.

The kinematic model provides an estimate based on the geometries of the towing vehicle and trailer, the steering angle, and the velocity. This estimate produces smoothly changing output, but drifts over time away from the actual trailer angle. The angle determination based on analysis of each video frame relative to a reference produces results that are subject to greater noise than the kinematic model, but are not subject to the cumulative drift.

Data from both approaches are combined in a prediction and correction filters. The trailer angle detection system's output comes from the smoothing filter, and is more accurate than the kinematic model and smoother than the raw measurements.

4A. Feature Detection and Tracking:

In order to track the movement of the trailer in video stream, the trailer angle detection system uses several algorithms developed for computer vision to calculate how much angular difference there is between the current video frame and the reference frame. To improve accuracy, the trailer angle detection system also implements a mechanism to update the reference frame as the trailer changes angle, giving the matching algorithm the most opportunity to find and track features on the trailer. The trailer angle detection system uses features that are a combination of patterns and key points to analyze the current video frame and find visual features that are distinctive enough to track from one frame to the next. The trailer angle detection system limits the detection of features within the dynamic field of view (FOV) of the camera.

4B. Feature Matching:

The algorithm takes the set of features separately detected in the current frame and the reference frame, and finds the correspondence. This produces a set of feature references that are deemed to have matched.

4C. Feature Filtering:

The features represented may be deemed to “match”, but some of them may not actually be part of the trailer, despite the use of the dynamic mask. To eliminate those feature matches that are not actually part of the trailer, some filtering steps are taken based on the behavior of trailer.

For example, the system may filter by conditions. One condition is based on the fact that trailer features will rotate around the hitch ball as the towing vehicle and trailer turn, whereas features not on the trailer (e.g., on the ground) move independently of the hitch ball. Another condition is that trailer features do not change much in size properties. Another condition is that matched features that are on the trailer will have similar vector angles or low angular differences in position between the current and the reference frame, while matched features not on the trailer will have random angular changes or differences or dissimilar angular differences of vectors of features over multiple frames of captured image data. For example, features determined on a trailer, as the trailer moves relative to the vehicle (such as during a turning maneuver of the vehicle and trailer) will have similar angular feature vectors (and thus the differences between the vector angles will be low and similar or non-random) in that the features move together relative to the vehicle, while features that are not indicative of features on the trailer, such as features of an object on the ground, will have dissimilar or random angular differences or changes as they move over multiple frames of captured image data, due to the non-uniform movement of the vehicle relative to the object, with the field of view of the camera changing relative to the object. The features that remain after filtering will be consolidated over the time. This improves the robustness of the system.

4D. Measurement of Angle:

For features that survive the filtering, the mean angular difference is calculated, resulting in a consensus angle that the frame differs from the reference frame. The absolute measured angle of the trailer is the consensus angle computed above, plus the offset angle for the current reference image. If at least one features survives the filtering, the frame is deemed a “good frame”, and the consensus angle is used to update the predictive filter.

4E. Multiple Reference Images:

To improve the accuracy of computed angles, it is advantageous to have a reference image of the trailer that is similar in angle to the current frame image. The trailer angle detection system implements a set of reference descriptors at fixed angles. The number of segments is a configurable parameter.

For example, FIG. 8 shows a set that consists of seven segments, each serving a range of 30 degrees. Optionally, the system may utilize a greater number of segments to enhance the angle accuracy.

As the vehicle turns and the trailer angle changes, the trailer angle detection system selects the reference descriptor for the segment within which the previous frame angle falls, making it the current descriptor, and using it for feature matching. The reference descriptor set data structure includes, in addition to the features and their locations, the angle of the frame image, known as the offset angle. This offset angle is near to the center angle of the segment. As the trailer angle changes, whenever the segment's center angle is crossed, the reference image data (from a reference frame of captured image data) for that segment are updated with the already computed feature data from the previous consolidated good frames of captured image data. This mechanism, which is periodically updating the reference image as the vehicle and trailer move, serves to improve the ability of the feature matcher to find matches, especially as lighting changes.

The system may utilize aspects of the trailering or trailer angle detection systems described in U.S. Pat. Nos. 9,085,261 and/or 6,690,268, and/or U.S. Publication Nos. US-2017-0254873; US-2017-0217372; US-2017-0050672; US-2015-0217693; US-2014-0160276; US-2014-0085472 and/or US-2015-0002670, and/or U.S. patent application Ser. No. 15/885,855, filed Feb. 1, 2018, and published on Aug. 2, 2018 as U.S. Publication No. US-2018-0215382, and/or Ser. No. 15,910,100, filed Mar. 2, 2018, and published on Sep. 6, 2018 as U.S. Publication No. US-2018-0253608, and/or U.S. provisional applications, Ser. No. 62/533,694, filed Jul. 18, 2017, Ser. No. 62/474,645, filed Mar. 22, 2017, which are hereby incorporated herein by reference in their entireties.

The camera or sensor may comprise any suitable camera or sensor. Optionally, the camera may comprise a “smart camera” that includes the imaging sensor array and associated circuitry and image processing circuitry and electrical connectors and the like as part of a camera module, such as by utilizing aspects of the vision systems described in International Publication Nos. WO 2013/081984 and/or WO 2013/081985, which are hereby incorporated herein by reference in their entireties.

The system includes an image processor operable to process image data captured by the camera or cameras, such as for detecting objects or other vehicles or pedestrians or the like in the field of view of one or more of the cameras. For example, the image processor may comprise an image processing chip selected from the EYEQ family of image processing chips available from Mobileye Vision Technologies Ltd. of Jerusalem, Israel, and may include object detection software (such as the types described in U.S. Pat. Nos. 7,855,755; 7,720,580 and/or 7,038,577, which are hereby incorporated herein by reference in their entireties), and may analyze image data to detect vehicles and/or other objects. Responsive to such image processing, and when an object or other vehicle is detected, the system may generate an alert to the driver of the vehicle and/or may generate an overlay at the displayed image to highlight or enhance display of the detected object or vehicle, in order to enhance the driver's awareness of the detected object or vehicle or hazardous condition during a driving maneuver of the equipped vehicle.

The vehicle may include any type of sensor or sensors, such as imaging sensors or radar sensors or lidar sensors or ladar sensors or ultrasonic sensors or the like. The imaging sensor or camera may capture image data for image processing and may comprise any suitable camera or sensing device, such as, for example, a two dimensional array of a plurality of photosensor elements arranged in at least 640 columns and 480 rows (at least a 640×480 imaging array, such as a megapixel imaging array or the like), with a respective lens focusing images onto respective portions of the array. The photosensor array may comprise a plurality of photosensor elements arranged in a photosensor array having rows and columns. Preferably, the imaging array has at least 300,000 photosensor elements or pixels, more preferably at least 500,000 photosensor elements or pixels and more preferably at least 1 million photosensor elements or pixels. The imaging array may capture color image data, such as via spectral filtering at the array, such as via an RGB (red, green and blue) filter or via a red/red complement filter or such as via an RCC (red, clear, clear) filter or the like. The logic and control circuit of the imaging sensor may function in any known manner, and the image processing and algorithmic processing may comprise any suitable means for processing the images and/or image data.

For example, the vision system and/or processing and/or camera and/or circuitry may utilize aspects described in U.S. Pat. Nos. 9,233,641; 9,146,898; 9,174,574; 9,090,234; 9,077,098; 8,818,042; 8,886,401; 9,077,962; 9,068,390; 9,140,789; 9,092,986; 9,205,776; 8,917,169; 8,694,224; 7,005,974; 5,760,962; 5,877,897; 5,796,094; 5,949,331; 6,222,447; 6,302,545; 6,396,397; 6,498,620; 6,523,964; 6,611,202; 6,201,642; 6,690,268; 6,717,610; 6,757,109; 6,802,617; 6,806,452; 6,822,563; 6,891,563; 6,946,978; 7,859,565; 5,550,677; 5,670,935; 6,636,258; 7,145,519; 7,161,616; 7,230,640; 7,248,283; 7,295,229; 7,301,466; 7,592,928; 7,881,496; 7,720,580; 7,038,577; 6,882,287; 5,929,786 and/or 5,786,772, and/or U.S. Publication Nos. US-2014-0340510; US-2014-0313339; US-2014-0347486; US-2014-0320658; US-2014-0336876; US-2014-0307095; US-2014-0327774; US-2014-0327772; US-2014-0320636; US-2014-0293057; US-2014-0309884; US-2014-0226012; US-2014-0293042; US-2014-0218535; US-2014-0218535; US-2014-0247354; US-2014-0247355; US-2014-0247352; US-2014-0232869; US-2014-0211009; US-2014-0160276; US-2014-0168437; US-2014-0168415; US-2014-0160291; US-2014-0152825; US-2014-0139676; US-2014-0138140; US-2014-0104426; US-2014-0098229; US-2014-0085472; US-2014-0067206; US-2014-0049646; US-2014-0052340; US-2014-0025240; US-2014-0028852; US-2014-005907; US-2013-0314503; US-2013-0298866; US-2013-0222593; US-2013-0300869; US-2013-0278769; US-2013-0258077; US-2013-0258077; US-2013-0242099; US-2013-0215271; US-2013-0141578 and/or US-2013-0002873, which are all hereby incorporated herein by reference in their entireties. The system may communicate with other communication systems via any suitable means, such as by utilizing aspects of the systems described in International Publication Nos. WO 2010/144900; WO 2013/043661 and/or WO 2013/081985, and/or U.S. Pat. No. 9,126,525, which are hereby incorporated herein by reference in their entireties.

Optionally, the vision system may include a display for displaying images captured by one or more of the imaging sensors for viewing by the driver of the vehicle while the driver is normally operating the vehicle. Optionally, for example, the vision system may include a video display device, such as by utilizing aspects of the video display systems described in U.S. Pat. Nos. 5,530,240; 6,329,925; 7,855,755; 7,626,749; 7,581,859; 7,446,650; 7,338,177; 7,274,501; 7,255,451; 7,195,381; 7,184,190; 5,668,663; 5,724,187; 6,690,268; 7,370,983; 7,329,013; 7,308,341; 7,289,037; 7,249,860; 7,004,593; 4,546,551; 5,699,044; 4,953,305; 5,576,687; 5,632,092; 5,708,410; 5,737,226; 5,802,727; 5,878,370; 6,087,953; 6,173,501; 6,222,460; 6,513,252 and/or 6,642,851, and/or U.S. Publication Nos. US-2014-0022390; US-2012-0162427; US-2006-0050018 and/or US-2006-0061008, which are all hereby incorporated herein by reference in their entireties. Optionally, the vision system (utilizing the forward viewing camera and a rearward viewing camera and other cameras disposed at the vehicle with exterior fields of view) may be part of or may provide a display of a top-down view or bird's-eye view system of the vehicle or a surround view at the vehicle, such as by utilizing aspects of the vision systems described in International Publication Nos. WO 2010/099416; WO 2011/028686; WO 2012/075250; WO 2013/019795; WO 2012/075250; WO 2012/145822; WO 2013/081985; WO 2013/086249 and/or WO 2013/109869, and/or U.S. Publication No. US-2012-0162427, which are hereby incorporated herein by reference in their entireties.

Changes and modifications in the specifically described embodiments can be carried out without departing from the principles of the invention, which is intended to be limited only by the scope of the appended claims, as interpreted according to the principles of patent law including the doctrine of equivalents. 

1. A vehicular trailer angle detection system, the vehicular trailer angle detection system comprising: a camera disposed at a rear portion of a vehicle, the camera viewing exterior and at least rearward of the vehicle; wherein the camera comprises an imaging array having at least one million photosensor elements arranged in rows and columns; wherein the camera is operable to capture frames of image data; an image processor operable to process frames of image data captured by the camera; wherein the vehicular trailer angle detection system determines, via processing by the image processor of captured frames of image data, features of a trailer present rearward of the vehicle and hitched to the vehicle; wherein the vehicular trailer angle detection system determines features of the trailer by determining, via processing by the image processor of captured frames of image data, features that have similar position changes between a current frame of image data captured by the camera and a previous frame of image data captured by the camera; wherein, responsive to movement of the trailer relative to the vehicle, and via processing by the image processor of captured frames of image data, the vehicular trailer angle detection system tracks determined features over multiple captured frames of image data for different positions of the trailer relative to the vehicle; and wherein the vehicular trailer angle detection system determines angle of the trailer relative to the vehicle based at least in part on tracking of determined features of the trailer over multiple captured frames of image data.
 2. The vehicular trailer angle detection system of claim 1, wherein the vehicular trailer angle detection system determines angle of the trailer without use of a target on the trailer.
 3. The vehicular trailer angle detection system of claim 1, wherein the vehicular trailer angle detection system determines a mean angular change of the determined features from a reference captured frame of image data to the current captured frame of image data, and wherein determination of the mean angular change of the determined features results in a determined change in trailer angle from the reference captured frame of image data.
 4. The vehicular trailer angle detection system of claim 3, wherein the vehicular trailer angle detection system determines angle of the trailer relative to the vehicle by combining the determined change in trailer angle with an offset angle for the reference captured frame of image data.
 5. The vehicular trailer angle detection system of claim 1, wherein the vehicular trailer angle detection system determines if the trailer is a trailer for which the vehicular trailer angle detection system is calibrated.
 6. The vehicular trailer angle detection system of claim 5, wherein the vehicular trailer angle detection system, responsive to determination that the trailer is not a trailer for which the vehicular trailer angle detection system is calibrated, collects data during a calibration drive.
 7. The vehicular trailer angle detection system of claim 6, wherein the calibration drive comprises (i) a straight portion where the vehicle tows the trailer in a forward direction along a straight path and (ii) a curved portion where the vehicle tows the trailer in the forward direction along a curved path.
 8. The vehicular trailer angle detection system of claim 7, wherein, during the calibration drive, the vehicle is driven along the straight path for at least a threshold period of time, and, after being driven along the straight path for at least the threshold period of time, the vehicle is turned such that the angle of the trailer relative to the vehicle exceeds ±5 degrees relative to a longitudinal axis of the vehicle.
 9. The vehicular trailer angle detection system of claim 8, wherein the threshold period of time is at least three seconds.
 10. The vehicular trailer angle detection system of claim 1, wherein the vehicular trailer angle detection system determines angle of the trailer relative to the vehicle independent of lighting conditions at the vehicle and trailer.
 11. The vehicular trailer angle detection system of claim 1, wherein the vehicular trailer angle detection system determines angle of the trailer relative to the vehicle independent of road surfaces along which the vehicle is towing the trailer.
 12. The vehicular trailer angle detection system of claim 1, wherein the determined features of the trailer are tracked over multiple captured frames of image data based at least in part on at least one vehicle parameter.
 13. The vehicular trailer angle detection system of claim 12, wherein the at least one vehicle parameter comprises vehicle speed.
 14. The vehicular trailer angle detection system of claim 12, wherein the at least one vehicle parameter comprises vehicle steering.
 15. The vehicular trailer angle detection system of claim 12, wherein the at least one vehicle parameter comprises (i) vehicle speed and (ii) vehicle steering.
 16. The vehicular trailer angle detection system of claim 1, wherein the image processor comprises a digital signal processor.
 17. A vehicular trailer angle detection system, the vehicular trailer angle detection system comprising: a camera disposed at a rear portion of a vehicle, the camera viewing exterior and at least rearward of the vehicle; wherein the camera comprises an imaging array having at least one million photosensor elements arranged in rows and columns; wherein the camera is operable to capture frames of image data; an image processor operable to process frames of image data captured by the camera; wherein the vehicular trailer angle detection system determines, via processing by the image processor of captured frames of image data, features of a trailer present rearward of the vehicle and hitched to the vehicle; wherein the vehicular trailer angle detection system determines features of the trailer by determining, via processing by the image processor of captured frames of image data, features that have similar position changes between a current frame of image data captured by the camera and a previous frame of image data captured by the camera; wherein, responsive to movement of the trailer relative to the vehicle, and via processing by the image processor of captured frames of image data, the vehicular trailer angle detection system tracks determined features over multiple captured frames of image data for different positions of the trailer relative to the vehicle; wherein the vehicular trailer angle detection system determines trailer length from a hitch ball of the vehicle to an axle of the trailer based on tracking of the determined features of the trailer over multiple captured frames of image data while the trailer is towed by the vehicle; wherein the vehicular trailer angle detection system determines angle of the trailer relative to the vehicle based at least in part on the determined trailer length; and wherein the vehicular trailer angle detection system determines angle of the trailer without use of a target on the trailer.
 18. The vehicular trailer angle detection system of claim 17, wherein the vehicular trailer angle detection system determines if the trailer is a trailer for which the vehicular trailer angle detection system is calibrated, and wherein the vehicular trailer angle detection system, responsive to determination that the trailer is not a trailer for which the vehicular trailer angle detection system is calibrated, collects data during a calibration drive, and wherein the calibration drive comprises (i) a straight portion where the vehicle tows the trailer in a forward direction along a straight path and (ii) a curved portion where the vehicle tows the trailer in the forward direction along a curved path.
 19. The vehicular trailer angle detection system of claim 18, wherein, during the calibration drive, the vehicle is driven along the straight path for at least a threshold period of time, and, after being driven along the straight path for at least the threshold period of time, the vehicle is turned such that the angle of the trailer relative to the vehicle exceeds ±5 degrees relative to a longitudinal axis of the vehicle.
 20. The vehicular trailer angle detection system of claim 19, wherein the threshold period of time is at least three seconds.
 21. The vehicular trailer angle detection system of claim 17, wherein the vehicular trailer angle detection system, responsive to determination of the trailer length, determines trailer parameters for the trailer and stores the determined trailer parameters for use in future trailer recognition.
 22. A vehicular trailer angle detection system, the vehicular trailer angle detection system comprising: a camera disposed at a rear portion of a vehicle, the camera viewing exterior and at least rearward of the vehicle; wherein the camera comprises an imaging array having at least one million photosensor elements arranged in rows and columns; wherein the camera is operable to capture frames of image data; an image processor operable to process frames of image data captured by the camera; wherein the vehicular trailer angle detection system determines, via processing by the image processor of captured frames of image data, features of a trailer present rearward of the vehicle and hitched to the vehicle; wherein the vehicular trailer angle detection system determines features of the trailer by determining, via processing by the image processor of captured frames of image data, features that have similar position changes between a current frame of image data captured by the camera and a previous frame of image data captured by the camera; wherein, responsive to movement of the trailer relative to the vehicle, and via processing by the image processor of captured frames of image data, the vehicular trailer angle detection system tracks determined features over multiple captured frames of image data for different positions of the trailer relative to the vehicle; wherein the determined features of the trailer are tracked over multiple captured frames of image data based at least in part on at least one vehicle parameter; wherein the vehicular trailer angle detection system determines angle of the trailer relative to the vehicle based at least in part on tracking of determined features of the trailer over multiple captured frames of image data; and wherein the vehicular trailer angle detection system determines angle of the trailer without use of a target on the trailer.
 23. The vehicular trailer angle detection system of claim 22, wherein the vehicular trailer angle detection system determines if the trailer is a trailer for which the vehicular trailer angle detection system is calibrated, and wherein the vehicular trailer angle detection system, responsive to determination that the trailer is not a trailer for which the vehicular trailer angle detection system is calibrated, collects data during a calibration drive, and wherein the calibration drive comprises (i) a straight portion where the vehicle tows the trailer in a forward direction along a straight path and (ii) a curved portion where the vehicle tows the trailer in the forward direction along a curved path.
 24. The vehicular trailer angle detection system of claim 23, wherein, during the calibration drive, the vehicle is driven along the straight path for at least a threshold period of time, and, after being driven along the straight path for at least the threshold period of time, the vehicle is turned such that the angle of the trailer relative to the vehicle exceeds ±5 degrees relative to a longitudinal axis of the vehicle.
 25. The vehicular trailer angle detection system of claim 24, wherein the threshold period of time is at least three seconds.
 26. The vehicular trailer angle detection system of claim 22, wherein the at least one vehicle parameter comprises vehicle speed.
 27. The vehicular trailer angle detection system of claim 22, wherein the at least one vehicle parameter comprises vehicle steering.
 28. The vehicular trailer angle detection system of claim 22, wherein the at least one vehicle parameter comprises (i) vehicle speed and (ii) vehicle steering.
 29. The vehicular trailer angle detection system of claim 22, wherein the vehicular trailer angle detection system determines trailer length from a hitch ball of the vehicle to an axle of the trailer based on tracking of the determined features of the trailer over multiple captured frames of image data while the trailer is towed by the vehicle.
 30. The vehicular trailer angle detection system of claim 29, wherein the vehicular trailer angle detection system, responsive to determination of the trailer length, determines trailer parameters for the trailer and stores the determined trailer parameters for use in future trailer recognition. 